The exponential increase in the demand for high-speed access to the Internet and other telecommunications services has created a significant demand for reliable equipment to provide such services. One of the most important classes of high-speed access communications equipment is digital subscriber line (DSL) equipment, used to offer a number related services collectively referred to as xDSL. The xDSL services include Asynchronous Digital Subscriber Line (ADSL), High Speed Digital Subscriber Line (HDSL), Symmetric Digital Subscriber Line (SDSL), and Very High Speed Digital Subscriber Line (VDSL). The xDSL services are provided using twisted pair copper loops commonly referred to as "subscriber loops". The xDSL transceivers typically convert a digital signal to be transmitted to an analog signal that is processed and coupled to the subscriber loop by a line driver.
In order to transmit signals at high speed, it is essential that the line driver introduce very little distortion in a signal transmitted on the subscriber loop. This is especially important if a multi-carrier modulation technique such as discrete multi-tone (DMT) modulation is used to transmit the signal. Energy conservation is also becoming increasingly important and it is therefore important that line driver circuitry dissipate as little waste energy as possible.
High efficiency line drivers are known in the art. Applicant's co-assigned and co-pending U.S. patent application Ser. No. 09/209,294 entitled DIGITAL SUBSCRIBER LINE DRIVE ARRANGEMENT WITH SELECTABLE VOLTAGE SUPPLIES was filed on Dec. 11, 1998. That application describes a technique in which a power supply for a DSL line driver is switched when an amplitude of the signal crosses a predefined threshold. Due to an inherent delay between switching on the power supply and an output of a drive voltage, the power supply must be switched on before the signal to be transmitted arrives at an input stage of the line driver. However, if the power supply is switched on too abruptly the switching mechanism may inject noise into the signal at the output of the driver. To avoid this problem, the slew rate of the switching mechanism is decreased. Consequently, the higher voltage supply must be turned on sooner, which results in less power savings because the higher voltage remains on longer.
Most xDSL signals have peaks that are nearly Gausian in their distribution. Consequently, very few peaks are at full scale. Therefore, if power supply switching is based on when an amplitude of the signal crosses a threshold, regardless of the peak amplitude of the signal, surplus power may be dissipated because the higher voltage power supply is kept on longer than required by the peak amplitude of the signal.
There therefore exists a need for a supply voltage control circuit for selecting one of 2N (N.gtoreq.1) power supplies to supply voltage to a line driver that is more energy efficient than control circuits described in the prior art.